Voltage-controlled spectral separation of light with liquid crystals

ABSTRACT

Light of different colors is produced by using as a &#39;&#39;&#39;&#39;diffraction grating&#39;&#39;&#39;&#39; a cell containing a liquid crystal material, with selectively variable fields being applied across the cell to present the different colors at a viewing station. Application is made to monochromators, symbol displays, and other devices.

U llllefl bIaIeS l Hedman, Jr. et al.

Sept. 11, 1973 Appl. No.: 242,709

U.S. Cl. 350/160 LC, 340/336, 350/l68, 356/100 Int. Cl. G02f l/l6 Fieldof Search 350/160 LC, 162 R, 350/168; 340/324 R, 336; 356/100 ReferencesCited UNITED STATES PATENTS 3/1970 Heilmeier et al. 350/160 LC X3,675,988 7/l972 Soref 350/160 l.C

OTHER PU BLlCATlONS Soref: Solid Facts about Liquid Crystals," LaserFocus, Vol. 6, pp. 45-49, Sept. 1970.

Grenbel et al.: Electrically Controlled Domains in Nematic LiquidCrystals, Applied Physics Letter, Vol, l9, pp. 213-215, Oct. l, 1971.

Carroll: Liquid-Crystal Diffraction Grating," Vol. 43, pp. 767-770.Jour. of App, Phys.. March. 1972.

Primary Examiner-Edward S. Bauer Au0rneyArmand G. Guibert et al.

[57] ABSTRACT Light of different colors is produced by using as adiffraction grating" a cell containing a liquid crystal ma erial, withselectively variable fields being applied across the cell to present thedifferent colors at a viewing station. Application is made tomonochromators, symbol displays, and other devices.

9 Claims, 10 Drawing Figures SEQUENCER 47 )V r.

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sum 1 or 3 FUNCTION GEN.

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CONVERTER TRIBUTOR EQUENCER CODE DIS

I I I CODECONVERTER 49 DISTRIBUTOR ENCER VOLTAGE-CONTROLLED SPECTRALSEPARATION OF LIGHT WITH LIQUID CRYSTALS BACKGROUND OF THE INVENTION Theinvention relates to improved electro-optical elements for producingdispersion of white light into its separate spectral components. Morespecifically, the invention relates to elements of that type which areelectrically controlled and which employ liquid crystals.

The term liquid crystals," as used hereinafter, refers to a class ofcompounds which show unusual dual characteristics in transition fromsolid to liquid. The transition occurs over a temperature range in whichthe compounds possess fluid flow characteristics of liquids yetdemonstrate anistropic optical properties expected only of crystallinesolids (see Molecular Structures and the Properties of Liquid Crystals,"by G. W. Gray, Academic Press, New York 1961 These compounds aregenerally organic, but the phenomenon has also been observed withsolutions involving inorganic materials. The transition region wheresuch peculiar characteristics are observed has generally been termed themesomorphic state or mesophase" and that term will be used hereinafterto define that state of any compound exhibiting the phenomenon.

There has been much research on such liquid crystals, particularly theirlight-scattering properties, as seen in the above-referenced book by G.W. Gray. Further, the light-scattering properties of such crystals havefound use in light valves (e.g., British Patent No. 41 L274) and displaydevices (e.g., U. S. Pat. No. 3,322,485, or Liquid-Crystal DisplayDevices," by G. Heilmeier, Vol. 22, No. 4, pp. 100-106, ScientificAmerican, April 1970). Diffraction of monochromatic light by nematicliquid crystal cells (nematic referring to mesophases of a knownthread-like" form) has been observed and studied (Light DiffractionPhenomena in an ac-Excited Nematic Liquid Crystal Sample, by Sun Lu andDerick Jones, J. Appl. Phys. 42, 1971, pp. 2138-2140).

To our knowledge, though prior art discloses that when subjected to afield above a threshold value, thin layers of liquid crystals producediffraction" patterns in light beams, it does not disclose what we haveobserved namely, that certain liquid crystals-cause the dispersion of abeam of polychromatic light into a plurality of spectra and particularcolors, all being angularly-displaced from the beam axis, the angulardisplacement of the spectra (or colors) increasing as the field strengthincreases above the threshold value, that is, the spectra (or colors)moving away from the beam axis and from each other as the field strengthincreases. Also, the presence of the colors, i.e., uniformly coloredbands, which we have observed is not discussed, nor is the observeddifferent spatial orientation of spectra of AC-excited liquid crystalcells as compared to that of spectra of DC-excited cells. Thus, bymerely varying the field, which may be a DC and/or an AC electric field,selection of desired portions of the dispersed light for use inmonochromators, changeable-color displays, and the like, may then beachieved quite simply. In the past, selective lighting with differentcolors required separate sources of different wavelengths, or requiredinterchangeable filters, or prisms relatively movable with respect to anobjective, together with mechanisms or circuits for bringing one oranother of the colorselection elements into play.

SUMMARY OF THE INVENTION The field may be generated by applying acontrollable variable source of voltage to transparent electrodeslocated on the walls of the cell. As the field is increased beyond thethreshold value, the dispersed beam moves further away from its initiallocation off the original path of the beam. Thus, with the observersstation at a fixed location, the entire dispersion can be moved past hisview by merely varying the strength of the field. In this manner, awhite or polychromatic beam of light can be separated into a continuousspectrum (and into particular components, as will be seen) andsuccessive wavelengths (or selected ones of the particular components)brought sequentially to the view of an observer.

If a monochromatic beam of light is produced by this technique, it canbe incorporated in a two-dimensional beam deflector or x-y displaycontrolled solely by varying AC and DC voltages applied across the celljointly.

Furthermore, if the transparent electrodes are in the form of symbols,upon changing the voltage applied to the electrodes, the color of thesymbols displayed at the viewing station (or even the presence of adisplay) may be selected at will. The change can be effected by the mereturn of a knob, as above, or by automatic switching between two or moresources of voltage if discrete color changes (or an on/offcharacteristic) are desired.

The invention thus obviates the need for a plurality of sources of lightbecause one source of white light is sufficient to produce any of thecolors of the complete spectrum. Also, it has the advantage that thereis no need for complex, weighty and expensive electromechanical drivesfor moving filters, eyepieces, or the like.

BRIEF DESCRIPTION OF THE DRAWING A detailed description of the inventionin connection with the accompanying drawing, in which:

FIG. 1 is a schematic showing the arrangement of the elements in a basicembodiment of the invention utilizing a cell containing a mesomorphiccompound as the light-dispersing member;

FIG. 2 is a cross-sectional view of the cell of FIG. 1;

FIG. 3 is a schematic showing an alternative embodiment of the inventionas a monochromator;

FIGS. 4a,b are a set of two schematics showing another embodiment of theinvention as a bi color display, e.g., red display in FIG. 4a and bluedisplay in FIG. 412;

FIG. 5 is an exploded view of the cell of FIG. 4a,b;

FIG. 6 is a schematic showing the elements of a typical spectraldispersion;

FIG. 7 is a schematic showing a further embodiment of the invention as abeam deflector; and

FIGS. 80,17 are a set of two schematics showing the displacement of themonochromatic beam in the embodiment of FIG. 7 according to the type ofvoltage applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An electro-optical apparatusbasic to the various embodiments of the invention will first bedescribed with reference to FIG. 1. In this figure a source of white orpolychromatic light 1, which may be an ordinary incandescent bulb or afluorescent lamp, is connected by means of leads 2 to a supply ofelectrical energy 3 (I I VAC, say). Daylight could be used in a knownfashion .if a more nearly white source of light was desired.

The luminous energy output of source 1 is passed through beam-formingmeans which may comprise an opaque shield 4 with a slit (or pinhole) 5in it, as seen in FIG. 1, but could also comprise a system of lenses ormirrors. The resulting beam of light 6 would normally follow thestraight-line path (beam 6) to a viewing station 8 (a ground-glassscreen, for example), even with an optically activatable cell 7interposed, so long as cell 7 is optically inactive.

As shown in FIG. 2, cell 7 comprises two plates 11 and 12, preferablytransparent (e.g., glass, quartz) and each having plane-opposed parallelfaces. Plates 11 and 12 are positioned opposite each other so that theiradjacent faces 13 and 14, respectively, are parallel. The distancebetween the inner faces 13 and 14 of the two plates is critical, as willbe explained below, and should be less than 25 microns (approximately 1mil), preferably being between 5 to 12 microns. The plates are preparedwith at least one electrically conductive region or strip 15 on innerface 13 of plate 11, and at least one conductive region or strip 16 onthe inner face 14 of plate 12. The conductive region or paths 15,16 onthe plates are also transparent. This may conveniently be accomplishedby depositing thin layers of indium oxide or tin oxide on the desiredregions of the inner faces 13,14 on plates 11,12. The two plates 11,12have their conductive regions positioned substantially in apposition. Anelectrical lead wire 17 is connected (e.g., by soldering or by use of adrop of silver paste) to the conductive path 15 on plate 11, and anotherelectrical lead wire 18 is connected to the conductive path 16 on plate12.

A field may be applied by connecting lead wires 17,18 to a variablesource of electrical potential 9 (FIG. 1). The source 9 may be directcurrent supply comprising a battery 19 and a variable resistance 20connected in series with cell 7 through the lead wires 17,18. (It mayalso comprise alone or in superposition a variable voltage source ofalternating current, such as a sine wave generator, as will be seen.)

The faces 13,14 form part of an inner chamber in cell 7 for containing athin film (see FIG. 2) of a lightdispersing medium 10, the transparentplates 11 and 12 being positioned apart for this purpose by means ofspacers (not shown in FIG. 2). The spacing may be maintained by clampingdevices, or alternatvely through a suitable frame member. For thelightdispersing medium 10, we have found that an organic nematicmesomorphic compound, known commerically as Licristal (TM) No. IV andproduced by E. Merck Chemical Works, Darmstadt, Germany under Cat. No.lOl05 (the commercial preparation actually being a mixture of the twoisomers of p-Butyl-p'methoxy-azoxybenzene, (C,H,C.,H,NONC,H,OCH ispreferable, although a similar mixture of N-(p-Ethoxybenzylidine)-p-n-butyl-aniline (50 percent) andN-(p-Methoxybenzylidine)-p-n-butylaniline (50 percent) this lastavailable as Eastman Kodak Cat. No. ll24V does exhibit the desiredphenomenon, but not as satisfactorily. In this respect, it is worthy ofnote that with both mixtures, the spacing was found to be important atleast within the range of voltages examined specifically, no effectbeing obtainable with either mixture when a spacing of about 25 microns(or greater) was attempted, and the optimum effect being obtained withspacings of about 3 to 12 microns. Further, it should be mentioned thatthe cell should preferably be given a treatment to pre-establish adesired orientation for the nematic molecules, this treatment comprisingrubbing the conductive surfaces along the length of the cell, say, (axisperpendicular to the paper in FIGS. 1, 3, 4, and 7) with a cloth or softpaper tissue, in known fashion. Lastly, it should be noted that thepreferred mesomorphic compound exhibits the desired mesohase in thetemperature range between 16C and 76C, these temperatures including roomtemperature and being above the melting point of the solid compound, butbelow the temperature at which the molten compound becomes isotropic. Itwill be understood that hereinafter when reference is made to amesomorphic compound, it is intended to signify that the compound is inthe particular temperature range in which the light-dispersing mesophaseis exhibited.

In operation (referring now to FIGS. 1 and 2), via screen 8 observer 22views light passing through transparent plates 11 and 12, includingtransparent conductive regions or electrodes 15,16. Light source 1 ispositioned so that a beam of light 6 is incident on the transparentplate 11. The angle between the incident light ray and plate 11 is notcritical, except insofar as it has a bearing on the location of theoutput of cell 7. When the electric field between the electrodes 15 and16 is substantially zero (variable resistance 20 being at maximum), theincident light is transmitted through cell 7 in a regular manner. Underthese circumstances, as shown by beam 6' in FIG. 1, only a fraction ofthe light reaches the eye of observer 22, whose line of sight is off thenormal to the transparent plates 11,12; transparent electrodes 15,16;the layer 10 of mesomorphic material; and screen 8. Accordingly, if nofield is applied, the observer sees nothing on ground-glass 8 other thanthe white beam 6' at the center line.

When a weak field is applied between electrodes 15 and 16 by slightlylowering the resistance 20 at the output of voltage source 19, theobserver sees no change initially. If the strength of the applied fieldis continuously increased, no change is visible on viewing screen 8until a certain threshold value is reached. This thresh old value forthe applied field differed for the particular mesomorphic compoundsmentioned and for particular distances between the two electrodes, butcorresponded to a gradient of at least V4 volt per micron. When thevoltage gradient applied between the two electrodes reaches thisthreshold value, there begins a change in the properties of layer 10located between the two electrodes and therefore subject to the field.As a result of this change, the incident light is diffracted and aplurality of spectra (or uniformly colored bands, as will be seen)appear, moving away from the center of groundglass 8 as the voltage isincreased, such that ultimately the remotest spectrum (or band) reachesthe eye of observer 22, as shown by ray 2!. Thus, an area of screen 8which was previously dark, then presents a full spectrum across thewidth of screen 8 (assuming slit 5 to be of sufficient length), theresidual ray 6' at the center line simultaneously changing color also. Asimilar set of spectra becomes visible on the other side of the centerline of screen 8, as shown by the rays 21' in FIG. 1.

A more detailed view of the dispersion obtained with a slit 5 in shield4 is shown in FIG. 6, where it is seen that upon passing the thresholdvalue there typically appear on screen 8 above center line CL, two fullbright spectra 61a and 61b together with three or four alternatingbright bands 62-65 of uniform color (hereinafter referred to asanomalous dispersion bands), first a cyan (blue-green) band 62, markedwith a C to identify its color, then a magenta band 63, marked M toidentify its color, and lastly, a yellow-green band 64 marked YGtogether with a red-orange band 65, marked RO, adjacent spectra andbands being separated from one another by dark areas 66. Though notdetailed in FIG. 6, the output is completely symmetrical about thecenter line CL, this being indicated by inclusion of a red-orange line65 near the bottom of screen 8 in FIG. 6. Also not shown are further,much dimmer, repetitions of the basic patterns located beyond thoseshown, i.e., off screen 8.

In the foregoing example, light source 1 was polychromatic, but if lightbeam 6 is made monochromatic (for instance, by insertion of a filter inthe path of beam 6, as will be seen later in connection with FIG. 7), itfollows that only one color can appear at screen 8 instead of the wholespectrum. The diffracted beam 21 will appear (other conditions nothaving changed) in the normal spectral position for that color, thoughthe rest of the spectrum is, of course, absent. Thus, a small voltagevariation, i.e., from threshold (or above) to below threshold, will besufficient to effect a change from the color display to no display atall, i.e., an on-off color display. Once the display is turned on, theangle 0 of the diffracted monochromatic beam can be electricallycontrolled, as previously described.

The dispersion pattern of FIG. 6 is shown as being striatedhorizontally, which is representative of the pattern obtained with DCvoltage biasing. lf AC voltage with a frequency above roughly Hz isapplied, the striations are rotated 90", being oriented vertically asfurther described below but with patterns otherwise identical (exceptfor differences in number of bands and color of the anomalous dispersionbands), and exhibiting similar increase in angular displacement as theamplitude of the AC voltage increases. It is worthy of note that thequality of the AC rotated display is frequency-sensitive, beginning atabout 10 Hz, but with best results being obtained at about 100-350 Hz,and dispersion no longer being obtained at 1000 Hz. There is also somebeam displacement with increasing frequency, but the effect is small ascompared to that obtained with increase in voltage.

While the exact reason for this light-dispersing phenomenon has not beendetermined, it would seem from our microscopic studies of the previouslyspecified mesomorphic compounds that groups of molecules exist which areparallel-oriented, but not linearly-ordered, i.e., they are off-set fromone another for the most part. These groups apparently change from theiroff-set positions to more nearly linearly-ordered positions when theelectrical field across the material reaches the threshold value, Thischange in alignment of molecular groups, all groups coming together toform long, rodlike, linearly ordered arrangements, is thought to resultin pseudo-gratings" with attendant light-dispersing properties. Withrespect to the different orientation observed with AC excitation atfrequencies above about 10 Hz, microscopic studies show that under theseconditions there are no long rods, but linearly ordered arrays ofsubstantially equal length segments of the rods, the gaps betweensegments being in alignment at right angles to the segments, such thatthe segments form rows. These rows are believed to account for therotation of the dispersed pattern obtained with AC excitation of thetype described.

A first application of the invention is shown in the embodiments of FIG.3, which presents schematically a monochromator utilizing the basicelements of FIG, 1, except for changes in voltage control 9 andsubstitution of an opaque screen 32 for frosted-glass screen 8 ofFIG. 1. Screen 32 has a narrow aperture 33 in it so as to restrict theoperators view to a suitably narrow region of one of the spectra 6la,b(the anomalous dispersion bands 62-65 previously described in connectionwith FIG. 6 being excluded, of course, unless those particularwavelengths are of interest). The voltage control 9 now consists of abias voltage supply 34 and a function generator 35. The former maycomprise a battery 19 and variable resistor 20, or a variabletransformer with 117 VAC input and a full-wave rectifying bridge on itsoutput, or other known sources of bias voltages (if desired, knownvoltage-limiting devices may be included to compensate for swings in thenominally 117 VAC input). The latter may comprise a sinewave generator(as previously mentioned) or a ramp function generator (such as, forinstance, the horizontal drive of an oscilloscope or a television set),or may comprise a step function generator providing equal increments ofvoltage at each step (for instance, similar to the vertical controls fora television raster). Bias voltage supply 34 is connected in series withfunction generator 35 by a lead 37 and connected to cell 7 by lead 17,as before, but lead [8 is now connected to function generator 35.Accordingly, if the cell 7 has a 6 micron space filled with Licristal(TM) No. IV, and there are selected a bias supply of about 30 volts, DC,and a ramp function swing of about 13 volts, and the aperture 33 is solocated as to define an angle 0 of about 20 with the beam 6,6, thenapplication of the ramp voltage superimposed on the bias voltage towhich cell 7 is subjected, will cause the spectrum visible throughaperture 33 (corresponding to the second spectrum 61b in FIG. 6), tomove progressively from red through yellow to blue and abruptly back tored. It will be obvious that if the output is to be restricted to aparticular wavelength, then the ramp function may be removed and biasvoltage supply 34 alone may be adjusted to vary the angle of diffractionof the desired wavelength until it matches the viewing angle establishedby the location of aperture 33, e.g., 20 in the above example.

On the other hand, if function generator 35 is a sine wave generator andthe cell 7 is biased with a constant DC voltage above threshold, then alow frequency (less than 10 Hz) field can be applied in addition to a DCfield, with the result that the spectra will swing slowly back and forthacross their original display positions.

' will be discussed with reference to FIGS. 7 and 8a,b.

FIG. 7 shows generally a composite of the optical structure of FIG. Iand the series-connected field generators of FIG. 3. For purposes ofsuch display, as is seen, there are needed light source 1, shield 4having a pinhole 5 in it, cell 7 and a screen 78 together with amonochromatic filter 71 preferably located in the path of theundiffracted beam 6, i.e., between light source 1 and cell 7. (A thin,collimated' monochromatic light beam suitable for this purpose couldalso be achieved, without shield 4, by using a helium/neon laser, as isknown from prior art cited earlier). Filter 71 could also be located inthe path of the diffracted light beam 72, i.e., between cell 7 andobserver 22, as is obvious. With such arrangement, observer 22 will seea spot 73 of monochromatic light on screen 78, which spot can belaterally displaced by provision of a field across cell 7 by connectinga variable source of potential 9 to the electrodes 15,16 (see FIG. 2) ofcell 7. If, now, source 9 comprises a variable source of DC potential 74connected in series with a source of AC potential 75, as shown in FIG.7, and in this embodiment the source of variable AC potential 75 havinga frequency greater than 10 Hz, but less that 1000 Hz (preferably about100350 Hz), then as DC potential alone is changed, spot 73 will bedisplaced substantially vertically along the axis 80 as shown by thespots 81, 81 in FIG. 8a and as the AC potential alone is changed, spot73 will be displaced at right angles to the DC displacement along theaxis 82 as shown by the spots 83,83 in FIG. 8a the amount ofdisplacement being dependent on the magnitude of the potential, in eachcase. The DC displacement has been shown along the vertical axis withthe AC displacement along the horizontal axis, but these directionsdepend upon the direction of rubbing used in the pre-treatment, asdescribed earlier, and there assumed to have been along the length ofthe cell. In the range of frequency mentioned, the AC potential whencombined with the DC potential, causes an orthogonal displacement asshown in FIG. 8b for the case of superposition of an AC signal on a DCsignal. From FIG. 8b it will be noted that spot 73 is not only displacedorthogonally, as desired, but is duplicated several times becauseaddition of AC potential causes spot 73 to split into mirror images 81L,81R and 81L, 81R, one set of spots 81L, 81R moving along the rotatedaxis 80 and the other set of spots 81R, 81L moving along the image axis"80a. This limitation can be avoided, as is obvious, by proper locationof screen 78 with respect to cell 7 such that the maximum changes in therespective voltages will cause displacements whereby only a single spot73 will fall within the confines of screen 78 (or a selected portion ofthat screen for example, the upper right quadrant, as shown in FIG. 7).It should be remarked that though a screen 78 for observation by thehuman eye is shown in FIG. 7, the receiving station could also be, forexample, a dense array of photo-cells for automatic supply of coordinateinformation directly to data-processing equipment, in known fashion.Similar remarks apply equally well to the basic embodiment of FIG. I andothers of its variations, of course.

A third application of the invention shown in FIGS. 40,!) and 5 is abi-color digital display (the number of colors being limited only forpurposes of the description). In this embodiment (see FIGS. 40,12), thesource I and the slit 5 in the shield 4 are so located that beam 6 formsan angle of roughly 30-45 with the normal to the plane of the liquidcrystal layer of a cell 47 similar to cell 7. In cell 47 the transparentelectrodes (see FIG. 5) on the inner surface of plate 51 (correspondingto plate 11 of FIG. 2) are in the shape ofdigits fonned from selectableelectrode segments 55, while the transparent electrodes 58 on the innersurface of plate 52 (corresponding to plate 12 of FIG. 2) arerectangular in shape and of a size approximately equal to the segmenteddigits. Each electrode 58 is located opposite a set of electrodesegments 55, a three-digit display being represented in FIG. 5 althougha greater or lesser number of digits could be displayed. As seen in FIG.4a, with variable resistor 20 set at position A, the particular voltageapplied sequentially and selectively as will be described to theelements of cell 47 causes the series of digits to appear to theobserver's eye in a particular color (blue, say, to represent a positivebalance, for instance), as represented schematically by a beam 45, allother beams representing the same digits in different colors beingintercepted by the interior of housing 42, as shown by beam 46 andresidual beam 6. For purposes of minimizing possibility of undesiredmultiple reflections, the interior of housing 42 should be covered witha light-absorbing material, such as a flat black point. If, on the otherhand, variable resistor 20 is set at position B (see FIG. 4b), thevoltage now applied to cell 47 causes the displayed digits to appear ina different color (red, say, to indicate a negative balance), previouslyintercepted beam 46 now passing to the observer's eye and beam 45 beingintercepted in its turn by the housing 42. It will be recognized that ifthe beams 45 and 46 comprise spectra such as the bands 6la,b of FIG. 6,then the displayed digits may not be absolutely uniform in color fromtop to bottom, the quality depending on the relative sizes and locationsof the various elements of the systems. If the beams 45,46 correspond tothe anomalous dispersion bands 62,63 or 64,65 (or even 62,65), then theuniformity of color of the display is assured, although the secificcolors may have to be passed through a modifying filter if the cyan,magenta, etc. colors are not satisfactory. The filter could be installedin aperture 39 of FIGS. 4a,b. It should also be pointed out that if thevoltage corresponding to position B is chosen to be such as to positiona dark area 66 in the field of vision defined by aperture 39, then oneagain has an on/off" color display. From the data given in thediscussion of the embodiment of FIG. 3, it is clear that a change ofabout 7 volts (or less, depending on whether the desired color is at thecenter or the edge of the spectrum) will suffice to change from thecolor display to no display at all.

It would also be possible (not shown, but known) to install a lenssystem between cell 47 and a screen similar to screen 8 in aperture 39to magnify smaller, hence more uniformly colored digital elements to adesired size for display on screen 8. Another possibility (not shown,but known) would be to use a cell 7 as a source of colored light fordirect illumination of a digital display cell 47' similar in structureto cell 47, but filled with one of the prior art nematic liquid crystalsexhibiting light-scattering properties as known from thepreviously-referenced Heilmeier article. The light-scattering displaycell 47' might be placed in aperture 39 of FIGS. 4a,b, for example. Thesource cell 7 would then be located such that the portion of thespectrum falling upon cell 47' would be substantially uniform, i.e.,cell 7 suffi ciently remote from cell 47' such that the heights of thedigits in the latter subtend an arc in the spectrum which is smallrelative to the are for the entire spectrum.

For display of variable numerals (FIGS. 4a,b and the electrodes 41 eachcorresponding to one order of the digital display in cell 47 may beconnected successively to lead 17 from one end of battery 19 throughsequencer 43 (FIGS. 4a,b), timed in synchronization with theserial-by-digit, parallel-by-bit output of a data source (not shown, buta shift register would be an example) supplying digital data in the formof the 1-2-4-8 code, say, on four leads 48, to a codeconverter/distributor 49. Device 49 has as another input the output ofvariable resistor 20, and by means of appropriate gating forming part ofdevice 49, supplies that voltage via leads 44 to like segments 55 in allorders of the segmental digital display. Only the selected segments 55opposite that electrode 58 connected in circuit to battery 19 bysequencer 43 will be optically active at any moment, of course. Further,the cycle time for sequencer 43 must be such that there isn't anynoticeable flicker. Other known approaches (such as decoding all digitsin parallel, thus obviating the need for sequencer 43) may also beapplied without departure from the spirit of the invention. It will beclear, of course, that device 49 could be omitted if the display doesnot change and also that automatic control could be substituted formanual control of variable resistor in known fashion.

From the foregoing description, it is evident that the basic inventionrelates to apparatus for separating a beam of polychromatic light intoits spectral components by dispersing the beam of the light with atransparent cell containing a mesomorphic material (liquid crystal).When the mesomorphic material is subjected to a field of a particularmagnitude, in particular to an electrical field which may be DC and/orAC, it exhibits light-dispersing characteristics. By providing a screenwith a slit as the viewing station and a variable voltage supplyconnected to electrodes in contact with the mesomorphic material as thefield generating means, the invention can be applied to a monochromator,a low voltage on-off color display, and in the case of monochromaticlight with a combined AC-DC field applied to the cell, a two-dimensionalbeam deflector is possible. Furthermore, use of a variable voltagesupply in conjunction with electrodes forming symbols or segments ofsymbols in the basic invention provides a selectable-color symboldisplay.

Although the specification described particular embodiments, and somevariations of these embodiments were mentioned, other modifications ofthe basic invention will be evident to those skilled in the art. Suchembodiments are therefore to be considered as merely exemplary, theintent being that the spirit and scope of the invention be limited onlyby the appended claims.

What is claimed is:

1. In apparatus for displaying information symbols comprising a sourceof polychromatic light, a dispersion member, a beam-defining membercooperating with said source in such a way that a beam of saidpolychromatic light will be dispersed into its spectral components bysaid dispersion member when a field of sufficient magnitude to causedispersion is applied to the dispersion member, and a viewing stationfor observing the dispersed light, said viewing station being nonalignedwith said beam so that light is not viewed when a field of magnitudeinsufficient to cause dispersion is applied to the dispersion member andsaid dispersion member being located between said beam-defining memberand said viewing station, the improvement wherein said dispersion membercomprises:

a cell having walls transparent to said light,

a mesomorphic material in said cell, said mesomorphic material havinglight-dispersion characteristics when subjected to a field of particularmagnitude, said dispersion comprising formation of a plurality ofspectra, displaced angularly from said beam and onto said viewingstation,

a source of electrical potential to generate a field of magnitudegreater than said particular magnitude,

means to subject said mesomorphic material to said field comprisingelectrically-conductive, light transmitting coatings on said cell walls,at least one of said coatings being in symbol form, and electricalconductors connecting said coatings in circuit with said source ofpotential, said spectra being displaced in an amount dependent upon themagnitude of said field, and

a variable control for said source of electrical potential, whereby saidsymbol form may be presented at said viewing station as a shapedselected portion of the dispersed light.

2. The combination defined in claim 1, wherein said coatings comprise aplurality of ordinal displays with a said symbol coating in each orderconsisting ofa plurality of segments, each segment having a leadselectably connectible in circuit with said source of potential.

3. The combination of claim 2, wherein there are two sources havingfirst and second potentials, and further including a two-positioncontrol operable to provide said first potential to said symbol coatingswhen said control is in one of said two positions and to provide saidsecond potential to said symbol coatings when said control is in theother of said two positions, said first and second potentials causingsaid symbol forms to be presented at said viewing station incorresponding different colors.

4. The combination of claim 3, wherein said first potential causesdisplay in a first anomalous dispersion band and said second potentialcauses display in a second anomalous dispersion band.

5. The combination of claim 3, wherein said sources of potentialcomprise alternating current sources, the frequency of said currentbeing between 10 Hz and 1000 Hz.

6. The combination of claim 3, wherein said coatings are on opposed onesof said cell walls, said walls being spaced apart less than 25 microns.

7. The combination of claim 6, wherein said mesomorphic material has anematic mesophase.

8. The combination of claim 7, wherein said nematic mesomorphic materialcomprises an isomer of p Butylp methoxy azoxy benzene.

9. The combination of claim 7, wherein said nematic mesomorphic materialcomprises a mixture of N-(p Methoxybenzylidine)-p n-butylaniline andN-(p Ethoxybenzylidine)-p n-butylaniline.

* i i i i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,758,195 Dated September 11, 1973 Clarence La Hedman, Jre,Karl-Dieter So Myrenne, Inv and Perrv H, Vartanian, Jr,

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Col. 3, line 19, change "6" to 6' Col. 10, line 39, delete "further"line 40, change "including" to said variable control comprises line 45,change "forms" to form Signed and fizzled this twenty-third 1 Of March I9 76 [SEAL] Arrest:

RUTH C. MASON 1 C MARSHALL DANN Arresting Officer Commissioner ofParentsand Trademarks

2. The combination defined in claim 1, wherein said coatings comprise aplurality of ordinal displays with a said symbol coating in each orderconsisting of a plurality of segments, each segment having a leadselectably connectible in circuit with said source of potential.
 3. Thecombination of claim 2, wherein there are two sources having first andsecond potentials, and further including a two-position control operableto provide said first potential to said symbol coatings when saidcontrol is in one of said two positions and to provide said secondpotential to said symbol coatings when said control is in the other ofsaid two positions, said first and second potentials causing said symbolforms to be presented at said viewing station in corresponding differentcolors.
 4. The combination of claim 3, wherein said first potentialcauses display in a first anomalous dispersion band and said secondpotential causes display in a second anomalous dispersion band.
 5. Thecombination of claim 3, wherein said sources of potential comprisealternating current sources, the frequency of said current being between10 Hz and 1000 Hz.
 6. The combination of claim 3, wherein said coatingsare on opposed ones of said cell walls, said walls being spaced apartless than 25 microns.
 7. The combination of claim 6, wherein saidmesomorphic material has a nematic mesophase.
 8. The combination ofclaim 7, wherein said nematic mesomorphic material comprises an isomerof p Butyl-p'' methoxy azoxy benzene.
 9. The combination of claim 7,wherein said nematic mesomorphic material comprises a mixture of N-(pMethoxybenzylidine)-p n-butylaniline and N-(p Ethoxybenzylidine)-pn-butylaniline.